This invention relates to computed tomography using helical scanning. More specifically, the invention relates to reducing "skew" image artifacts resulting from tomographic reconstructions of projection data acquired in a helical scan.
As used herein, computed tomography shall refer both to tomography using "transmission imaging" that is, detecting radiation transmitted through the body being imaged, and "emission imaging", detecting radiation emitted from the body being imaged, e.g., such as that being emitted by radio pharmaceutical isotopes.
In a transmission imaging computed tomography system, an x-ray source is collimated to form a fan beam with a defined fan beam angle. The fan beam is orientated to lie within the x-y plane of a Cartesian coordinate system, termed the "imaging plane", and to be transmitted through an imaged object to an x-ray detector array oriented within the imaging plane. The detector array is comprised of detector elements which each measure the intensity of transmitted radiation along a ray projected from the x-ray source to that particular detector element. The intensity of the transmitted radiation is dependent on the attenuation of the x-ray beam along that ray by the imaged object. The detector elements can be organized along an arc each to intercept x-rays from the x-ray source along a different ray of the fan beam.
The x-ray source and detector array may be rotated on a gantry within the imaging plane and around the imaged object so that the angle at which the fan beam intersects the imaged object may be changed. At each gantry angle, a projection is acquired comprised of the intensity signals from each of detector elements. The gantry is then rotated to a new angle and the process is repeated to collect a number of projections along a number of gantry angles to form a tomographic projection set.
The acquired tomographic projection sets are typically stored in numerical form for computer processing to "reconstruct" a slice image according to reconstruction algorithms known in the art. The reconstructed tomographic images may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera.
In so called "fourth generation" transmission tomography systems, the detector array remains fixed and is expanded arcwise around the imaged object to subtend 180 degrees plus the fan beam angle or more of arc. In such systems, only the x-ray source is rotated to acquire the tomographic projection set.
Emission computed tomography may be performed in a similar manner. Briefly, a set of detectors are again rotated around the imaged object within an imaging plane. The detectors receive radiation not from an external x-ray source, but rather from radioactive isotopes within the object itself. The radiation received by the detectors reveals the relative concentrations of the radioactive source within the object being imaged. The detector array receives a different projection as its position is moved to different angles with respect to the imaged object all within the imaging plane.
In either emission or transmission computed tomography the detector array may be rectilinear rather than arcuate. The portions of the tomographic system that rotate, whether x-ray source, detector, or both shall be termed the gantry.
A typical computed tomographic study entails the imaging of a series of slices of an imaged object with the slices displaced incrementally along the z-axis, which is perpendicular to the x and y axes, so as to provide a third spatial dimension of information. A radiologist may visualize this third dimension by viewing the slice images in order of position along the z-axis, or the numerical data comprising the set of reconstructed slices may be compiled by computer programs to produce shaded, perspective representations of the imaged object in three dimensions.
As the resolving power of computed tomography methods increases, additional slices are required in the z-dimension. The time and expense of a tomographic study increases with the number of slices required. Also, longer scan times increase the discomfort to the patient who must remain nearly motionless to preserve the fidelity of the tomographic reconstructions. Accordingly, there is considerable interest in reducing the time required to obtain a slice series.
The time required to collect the data for a series of slices depends on four components: a) the time required to accelerate the gantry to scanning speed, b) the time required to obtain a complete tomographic projection set, c) the time required to decelerate the gantry and d) the time required to reposition the patient in the z-axis for the next slice. Reducing the time required to obtain a full slice series may be accomplished by reducing the time required to complete any of these four steps.
The time required for acceleration and deceleration of the gantry may be avoided in tomographic systems that use slip rings rather than cables to communicate electrical power and signals to and from the gantry. The slip rings permit continuous rotation of the gantry. Henceforth, unless otherwise stated, it will be understood that the systems described herein are equipped with slip rings or the like and are capable of continuous gantry rotation.
The time required to acquire the tomographic data set is more difficult to reduce. Present CT scanners require on the order of two seconds to acquire the projection set for one slice. This scan time may be reduced by rotating the gantry at a higher speed, but a higher gantry speed, in general, will reduce the signal-to-noise ratio of the acquired data by the square root of the factor of rotational rate increase. This may be overcome to some extent in transmission tomography devices by increasing the radiation output of the x-ray tube, but is subject to the power limits of such devices.
A reduction in patient repositioning time may be accomplished by translating the patient in the z-axis synchronously with the constant rotation of the gantry. The combination of constant patient translation along the z-axis during the rotation of the gantry and acquisition of projection data has been termed "helical scanning" and refers to the apparent path of a point on the gantry with respect to a reference point on the imaged body. As used herein, "helical scanning" shall refer generally to the use of continuous translation of the patient or imaged object during the acquisition of tomographic imaging data, and "constant z-axis scanning" shall refer to the acquisition of the tomographic data set without translation of the patient or imaged object during the acquisition period.
Referring to FIGS. 2 and 3, the motion of the gantry for a constant z-axis scan and a helical scan, respectively, are depicted The vertical axis on both figures indicates the relative z-axis position of the imaged object with respect to the imaging plane of the tomographic system, and the horizontal axis of both charts shows the gantry rotational angle .theta.. It will be understood that for constant gantry rotational speed, the horizontal axis also represents time.
Referring to FIG. 2, in a constant z-axis scan, each tomographic projection set may be acquired over 360.degree. and accordingly the horizontal axis on each chart has been marked to indicate the start and end of adjacent tomographic projections sets intervals of 360.degree.. The solid line on each chart indicates the relative position of the imaged object with respect to the imaging plane and is denoted the scan path. As indicated, the imaged object is held stationary so that the image plane aligns with a slice place, established with respect to the imaged object, when the projection set is acquired. After the tomographic projector set for a slice plane is acquired, the image object is moved to the next slice plane during a repositioning period.
This differs from the helical scan path shown in FIG. 3 where the z-axis position of the imaged object with respect to the imaging plane has a constant velocity during the acquisition of each tomographic projection set. Accordingly, the scan path is a sloped line. The slope of the scan path for helical scanning will be referred to as the scanning pitch.
Continuous translation of the imaged object during scanning shortens the total scanning time required for the acquisition of a given number of slices. Nevertheless, helical scanning as depicted in FIG. 3 introduces certain errors with regard to the data in the acquired projection sets. The mathematics of tomographic reconstruction assumes that the tomographic projection set is acquired along a constant z-axis slice plane, as indicated by the horizontal slice plane lines in FIG. 3. The helical scan path of FIG. 3 clearly deviates from the horizontal lines of the constant z axis slice planes.
Referring to FIG. 4 the chart of the helical scan path of FIG. 3 is shown as modified by mapping .theta. values of greater than 360.degree. over corresponding .theta. values from 0 to 360.degree. so as to emphasize the periodicity of the gantry motion in .theta.. This representation will be termed a "superimposed" scan path representation.
The deviation of the helical scan path from the slice plane results in image artifacts in the reconstructed tomographic image. The severity of the image artifacts depends generally on the "helix offset", indicated as the difference between z arguments of the scanned data and the z axis value of the desired slice plane and shown in FIG. 4. The helix offset error for a given scan path depends on the value of .theta. and is shown in FIG. 4 for .theta..dbd..theta.'. The errors resulting from helical scanning will be referred to as "skew" errors.
Several methods have been used to reduce skew errors in helical scanning. A first approach disclosed in U.S. Pat. No. 4,630,202 issued Dec. 16, 1986, reduces the pitch of the helical scan and then averages the projection data of consecutive 360.degree. tomographic projection sets. The effect is equivalent to using a detector array which has both a larger width along the z axis, and which travels more slowly along the z-axis, i.e. with a lesser helical pitch. Skew errors are reduced using this method, but at the expense of requiring additional scanning time as is necessitated by the lower helix pitch. Thus, this method reduces, to some extent, the advantages to be gained by helical scanning.
Skew errors at the ends of the tomographic projection set may be reduced in conjunction with this approach by changing the weighting of the last and first projections of the consecutive 360.degree. tomographic projection sets in the "averaging" process to give greater weight to the projection closest to the slice plane.
A second approach disclosed in U.S. Pat. No. 4,789,929 issued Dec. 6, 1988, also applies weighing to the projections of combined, consecutive 360.degree. tomographic projection sets, but the weighting is a function of the helical offset of each projection at the given .theta.. This interpolation approach generally reduces skew image but is prone to errors if the density of the imaged object changes rapidly along the z direction.